290 ISLAND LIFE 



coast, and even from the West Indian Islands and Gulf of 

 Mexico. As in the case of the birds, the several islands 

 differ considerably in their native plants, many species 

 being limited to one or two islands only, while others 

 extend to several. This is, of course, Avhat might be ex- 

 pected on any theory of their origin ; because, even if the 

 whole of the islands had once been united and afterwards 

 separated, long continued isolation would often lead to the 

 differentiation of species, while the varied conditions to be 

 found upon islands differing in size and altitude as well as 

 in luxuriance of vegetation, would often lead to the ex- 

 tinction of a species on one island and its preservation on 

 another. If the several islands had been equally well 

 explored, it might be interesting to see whether, as in the 

 case of the Azores, the number of species diminished in 

 those more remote from the coast ; but unfortunately our 

 knowledge of the productions of the various islands of the 

 group is exceedingly unequal, and, except in those cases 

 in which representative species inhabit distinct islands, we 

 have no certainty on the subject. All the more interesting 

 problems in geographical distribution, however, arise from 

 the relation of the fauna and flora of the group as a whole to 

 those of the surrounding continents, and we shall therefore 

 for the mo.st part confine ourselves to this aspect of the 

 question in our discussion of the phenomena presented by 

 oceanic or continental islands. 



Condudi7ig Remarks. — The Galapagos offer an instructive 

 contrast with the Azores, showing how a difference of con- 

 ditions th^t might be thought unimportant may yet pro- 

 duce very striking results in the forms of life. Although 

 the Gala])agos are much nearer a continent than the 

 Azores, the number of species of plants common to the 

 continent is much less in the former case than in the 

 latter, and this is still more prominent a characteristic of 

 the insect; and the bird faunas. This difference has been 

 shown to depend, almost entirely, on the one archipelago 

 being situated in a stormy, the other in a calm portion of 

 the ocean; and it demonstrates the preponderating im- 

 portance of the atmosphere as an agent in the dispersal of 

 birds, insects, and plants. Yet ocean-currents and surface- 



